Re: IRQ on M68K

[Peter Jeremy <peter.jeremy@auss2.alcatel.com.au>]

Juergen Suessmaier <juergen@ffab.tide.ti.com> wrote:
>To my knowledge it isn't possible to write interrupt routines with
>the GNU compiler,
At least for the M68K.  There are function attributes to define a
function as an interrupt handler for the H8/300 and M32R processors
(I don't know why they use different keywords though).  Feel free
to implement the same for the M68K.

>writing a small assembly routine that pushes all registers onto the
>stack, calls the C routine, pops all registers back from stack
Note that you don't need to save all registers, just those that
aren't saved during a normal function call.  Assuming no FP code
within the interrrupt handler, the following is sufficient:

IrqAsm:  movem.l %d0-%d1/%a0-%a1,-(%a7)
         jsr     handleInterrupt
         movem.l (%a7)+,%d0-%d1/%a0-%a1
         rte

Note that with care, it might be possible to use gcc's asm() directive
to turn a normal function into an interrupt handler.  This approach is
likely to be fairly fragile (ie you'll need to check the generated
assembler to make sure the code is still correct).

>Of course you can now call handleInterrupt() directly from your main
>C program, as long as you don't expect any reentrance problems
This also applies to any functions called within your interrupt handler.
It's worthwhile carefully studying the fine print on any library calls
you make - you definitely don't want to be trying to debug problems
caused by nested calls of non-reentrant functions.

Peter
--
Peter Jeremy (VK2PJ)                    peter.jeremy@alcatel.com.au
Alcatel Australia Limited
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